The FCC has adopted several initiatives designed to improve the handling of 911 calls from wireless/mobile sources. One such initiative includes a requirement for locating the wireless unit originating the call, within a set of predefined limits. For example, a first phase of the scheduled improvements requires that cell phones making an emergency call using the 911 system be locatable within 100 meters of their actual location at least 67 percent of the time, and within 300 meter of their actual location at least 95 percent of the time. A further phase of the scheduled improvements requires that these limits improve even more, such that the cell phones making an emergency call could be located within 50 meters/67 percent of the time and 150 meters/95 percent of the time.
One suggestion for meeting the scheduled FCC initiatives involves adding GPS location capability to each phone. However, this suggestion can be relatively costly, and would require a substantial change to many already existing phone designs. A further suggestion for meeting the scheduled FCC initiatives involves extracting time-of-arrival information from several base stations from which the mobile handset can collect bursts of data. The time-of-arrival data is then sent to the network, where the location of the handset is determined.
However one of the difficulties of using time-of-arrival information received from multiple base stations includes the fact that individual base stations are not generally time synchronized. At least one solution to the lack of time synchronization has included the use of Location Measurement Units (LMUs), which similarly receive time of arrival information from the same base stations and compute correction information based upon the measured values and generally known fixed locations.
A further complication to determining or extracting accurate and reliable time-of-arrival values includes the fact that cellular signals do not always arrive at the mobile subscriber through a line of sight path. Often times the signal received has been reflected off of one or more elements. In some instances the elements are man-made, like a building. In other instances the elements can occur naturally, for example a tree or a mountain. However generally the reception of wireless signals via indirect paths is generally more pronounced in urban environments, where a large percentage of the wireless communication traffic is occurring, and there is a high density of objects capable of reflecting a signal.
At least one prior reference attempts to address some of the problems associated with obtaining time arrival information, namely, Villier et al., U.S. patent application Ser. No. 09/468,998, entitled “Method and System for Estimating a Time of Arrival of a Radio Signal”, assigned to Motorola, the disclosure of which is incorporated herein by reference. In Villier et al. the time of arrival values corresponding to the line of sight paths are identified, by taking a subset of the total number of time of arrival values, which fall within a predetermined percentage range. In some instances this corresponds to the lowest time of arrival values.
However a further difficulty in determining accurate time of arrival information includes the fact that the reference clock for each base station is allowed to drift. Therefore any correction information received from a Location Measurement Unit is generally only good for data bursts that are received at the same time or proximate to the time that the correction information is determined.
Consequently, there is a need for a method and apparatus for generating time of arrival estimates, which take into account any drift in the reference clock of the base station.